Dataset Overview : This data set contains data acquired by the Galileo Magnetometer during the Interplanetary Cruise to Jupiter. The data are at varying resolution depending on the averaging constant applied by the instrument. These data have been fully processed to remove instrument response function characteristics. The data are provided in physical units (nanoTesla) and in 2 coordinate systems. This set of data files contains data in Inertial Rotor Coordinates (IRC: despun spacecraft). These files contain some of the data processing parameters from the AACS system as well as the sensor zero level for the spin aligned sensor. The data are also being archived in RTN coordinates which are provided on this CDROM as a separate dataset. Trajectory have been provided in heliographic coordinates as a separate dataset. In order to provide a more complete solar wind dataset the high resolution magnetometer data from the Earth1 encounter, Earth2 encounter and Ida encounter was averaged down to 16 minute resolution and included with this interplanetary cruise data. The 16 minute averages are derived from the following datasets: GO-E-MAG-3-RDR-E1-HIRES-V1.0 GO-E-MAG-3-RDR-E2-HIRES-V1.0 Data inside the earth's bowshock was excluded from the 16 min averages. Data Columns (IRC): sc_clk S/C clock counter given in the form rim:mod91:mod10:mod8 time S/C event time (UT) given in the form yyymmddTZ Bx_sc Magnetic field X component in S/C (IRC) coordinates  By_sc Magnetic field Y component in S/C (IRC) coordinates  Bz_sc Magnetic field Z component in S/C (IRC) coordinates  Bmag |B| Magnitude of B  zoff Offset subtracted from the z-sensor avg_con Averaging Constant (Rims) rotattd Rotor attitude declination (EME-50) rotattr Rotor attitude right ascension t Rotor twist angle (EME-50) minus Rotor spin angle(IRC coordinates) Data Acquisition : The data for this dataset were acquired as part of the normal instrument calibration activities associated with the cruise to Jupiter. As such, the instrument was commonly configured in modes which required calibration even though they may not have been the optimal mode for science data acquisition. The Galileo magnetometer has 8 possible acquisition configurations (modes). There are two sensor triads mounted 7 and 11 meters from the rotor spin axis (inboard and outboard) along the boom. Each of the sensor triads has two gain states (high and low). In addition, the sensor triads can be 'flipped' to move the spacecraft spin-axis aligned sensor into the spin plane and visa versa. Please see the instrument description for full details on the instrument, sensors, and geometries. The combinations of sensor, gain state, and flip direction form modes. _____________________________________________________________________ Mode Characteristics _____________________________________________________________________ Mode Name Acronym range quantization _____________________________________________________________________ Inboard, left, high range* ILHR +/- 16384 nT 8.0 nT Inboard, right, high range* IRHR +/- 16384 nT 8.0 nT Inboard, left, low range* ILLR +/- 512 nT 0.25 nT Inboard, right, low range* IRLR +/- 512 nT 0.25 nT Outboard, left, high range* ULHR +/- 512 nT 0.25 nT Outboard, right, high range* URHR +/- 512 nT 0.25 nT Outboard, left, low range* ULLR +/- 32 nT 0.008 nT Outboard, right, low range* URLR +/- 32 nT 0.008 nT __________________________________________ s/c clock date/time mode __________________________________________ 00104651:00:0 89-353/20:53 ULLR 00120107:00:0 89-364/17:21 URHR 00120117:00:0 89-364/17:31 IRHR 00120150:00:0 89-364/18:04 IRLR 00120178:00:0 89-364/18:33 ILLR 00120199:00:0 89-364/18:54 ILHR 00120209:00:0 89-364/19:04 URHR 00120231:00:0 89-364/19:26 URLR 00120283:00:0 89-364/20:19 ULLR 00120313:00:0 89-364/20:49 ULHR 00120492:00:0 89-363/23:50 ULHR 00572976:00:0 90-040/03:00 ULHR 00572976:00:0 90-316/17:00 ULLR 00578673:00:0 90-320/17:00 URLR 00586204:00:0 90-325/23:55 URHR 00592915:00:0 90-330/17:01 ILLR 00597439:00:0 90-333/21:15 IRLR 00610156:00:0 90-342/19:33 IRHR 00610509:00:0 90-343/01:30 IRLR 00615701:00:0 90-346/17:00 URLR 00618550:00:0 90-348/17:00 URHR 00624261:00:0 90-352/17:15 ULHR 01003052:72:8 90-253/16:36 ULLR 01600269:00:0 92-308/00:48 ULLR 01605562:00:0 92-311/18:10 ILLR 01629549:00:0 92-328/14:14 IRLR 01632087:00:0 92-330/09:00 ULLR 01635479:00:0 92-332/18:23 URHR 01638675:00:0 92-335/00:01 URLR 01639786:00:0 92-335/18:45 IRLR 01639974:00:0 92-335/21:55 URLR 01640958:00:0 92-336/14:30 IRLR 01641285:00:0 92-336/20:40 URHR 01641351:00:0 92-336/21:07 IRHR 01641407:00:0 92-336/22:15 ILHR 01641465:00:0 92-336/23:02 URLR 01642481:00:0 92-337/16:22 ULHR 01644143:00:0 92-338/20:10 IRLR 01646048:00:0 92-340/04:16 ILLR 01649533:00:0 92-342/15:00 ULLR 01650524:00:0 92-343/07:42 ULHR 01650907:00:0 92-343/14:09 ILHR 01651026:00:0 92-343/16:10 ULHR 01656418:00:0 92-347/11:01 ILLR 01659057:00:0 92-349/07:30 ULLR 02271339:18:2 94-048/05:34 URLR 02378309:54:6 94-123/08:13 ULLR 02618987:18:2 94-292/08:04 URLR 02702827:18:2 94-351/04:56 ULLR * Range is the opposite of gain - high range equals low gain Coordinate Systems : The data are being archived in two coordinate systems. The first coordinate system is referred to as inertial rotor coordinates (IRC). This coordinate system has the Z axis along the rotor spin axis, positive away from the antenna and the X and Y axes lies in the rotor spin plane. In a crude sense, when the spacecraft is far from Earth, +X points south, normal to the ecliptic plane, positive Y lies in the ecliptic plane in the sense of Jupiter's orbital motion and positive Z is in the anti-earth direction. The spacecraft antenna (negative Z direction) is kept earthward pointing to about +/- 10 deg accuracy. The second coordinate system is RTN coordinates. This system places Br along the Sun-spacecraft line with the positive direction away from the sun. Bn is along the direction of the Sun spin axis crossed with the radial axis. Btangential is in the direction of Bn crossed with the radial axis. Onboard Spacecraft Processing : The majority of the processing of the interplanetary cruise data is done onboard the spacecraft. The following procedures take place onboard. 1) Data is recursively filtered and decimated to 4.5 vectors/second 2) Sensor zero levels are subtracted and gains are applied 3) Data is transformed from a sensor coordinate system to a spacecraft coordinate system using an orthogonality matrix 4) Data are decimated again to 2/3 second resolution 5) Data are despun into inertial rotor coordinates 6) A recursive filter is applied and data are decimated to a variable resolution These steps are explained in further detail below. 1) Recursive filter and decimation The Galileo magnetometer samples the magnetic field 30 times per second. These highest rate samples are recursively filtered and then resampled by the instrument at 4.5 vectors per second using a 7,7,6 decimation pattern. Recursive Filter B(t) : 1/4 Bs(t) + 3/4 B(t-1) B : output field Bs : input field measured by the sensor t : sample time The pattern is generated by doubling the spacecraft clock modulo 10 counter and then applying the decimation scheme. This gives 3 vectors every spacecraft minor frame (about 2/3 second) which are sampled unevenly. The first vector in a minor frame is sampled approximately 0.200 seconds after the last vector in the preceding minor frame. The other two samples are taken approximately 0.233 seconds apart. 2) The data was processed onboard the spacecraft using a fixed zero level which was provided at the start of each data acquisition interval. These zero levels were determined using various methods and were periodically updated as significant changes in level were noted. Knowledge of the two spin plane sensor zero levels is not critical. The data is generally averaged over a long enough interval that any error in the zero levels is removed. Knowledge of the spin aligned sensor zero level is critical. However, this can be corrected for once the data has been received on the ground and is discussed below under Ground Data Processing. 3) Orthogonality matrix applied: The determination of a calibration matrix is too complex to describe here. The method employed has been well described in [KHURANAETAL1996]. 4) The data is decimated to 2/3 second resolution by keeping only the first minor frame of data. 5) Despinning data to inertial rotor coordinates: The nominal transformation to IRC from SRC is (Bx) ( cos(theta) -sin(theta) 0 ) (Bxs) (By) : ( sin(theta) cos(theta) 0 ) (Bys) (Bz) ( 0 0 1 ) (Bzs) Where s denotes spinning coordinates and theta is the rotor spin angle. 6) Recursive filter and decimation: A recursive filter is applied in in the following manner. Recursive Filter B(t) : 1/C Bs(t) + (1-1/C) B(t-1) B : output field Bs : input field measured by the sensor t : sample time C : averaging constant The averaging constant is variable and is given at the start of each data acquisition period. The data is then decimated. There is a one to one correspondence between the averaging constant used and the decimation factor as outlined below. Time per sample(Rims) Averaging constant Decimation Factor ------------------------------------------------------------------- 1 1024 1 2 512 2 4 256 4 8 128 8 16 64 16 32 32 32 64 16 64 128 8 128 256 4 256 512 2 512 1024 1 1024 Ground Data Processing : Once the data is on the ground there is very little that can be done to improve the onboard processing because of the heavy filtering and decimation. The following procedure is done on the ground 1) Data is converted to nanoTesla 2) The zero level of the spin aligned sensor is corrected. 3) Timing is set to account for a phase delay 4) Rotation is performed to account for phase delay 5) Data are transformed to RTN coordinates to produce the RTN dataset. These steps are outlined in greater detail below. 1) Conversion to nanoTesla simply requires dividing the instrument data numbers by a constant scale factor. For the inboard high range (low gain) mode the scale factor is 2. For the inboard low range and outboard high range, the scale factor is 64. The outboard low range data has a scale factor of 1024. 2) The zero level of the spin axis aligned sensor was determined by several means. One method is to take 25 day averages of the data from the spin aligned sensor. The basis of this method is that if the spacecraft is relatively close to the ecliptic plane, over one solar rotation the average of the suns magnetic field in the spacecraft z-direction is zero. Therefore whatever remains after averaging is taken to be an offset. The second method is dependent on the four quadrant structure of the solar wind. For this method we look only at points where By(IRC coordinates) crosses from positive to negative values. Here we assume that based on the four quadrant structure, as we pass from quadrant to quadrant By(IRC) will change signs and Bz(IRC) will go to zero. The values of Bz(IRC) where By(IRC) crosses zero are then averaged over a fifty day period. For the majority of the cruise data the instrument was configured in the ULLR mode. This mode puts sensor 3 along the spin axis. After looking at the results of these two method it became apparent that the spin aligned sensor 3 offset had a substantial drift that was correlated to the change in temperature. The offset determinations for sensor 3 were fit to temperature. It was found that two different fits were necessary. For temperatures greater than 272K a straight line fit was used. zoff : -17.60926 + .02608*(Temp) zoff : Offset for sensor three [nT] Temp : Temperature at the outboard sensor [K] For temperatures less than 272K a second fit was determined. zoff : 22.9016799 + (2511.27490/Temp) - (3176088.25)/(Temp*Temp) zoff : Offset for sensor three [nT] Temp : Temperature at the outboard sensor [K] A second instrument configuration commonly used during the cruise to Jupiter was the URLR mode. This mode places sensor2 along the spin aligned axis. For unknown reasons this sensor was not as susceptible to changes in temperature and showed no strong zero level drifts over time. A fixed offset of 2.75574nT was removed from sensor 2. 3) In order to account for the long recursive filters applied the timing of the data is corrected to be: rim:rim + (avg_con - (rim % avg_con)) +1/e*avg_con rim : starting rim of the averaging interval avg_con : length of the averaging interval(rims) 4) There is a phase delay associated with the Analog to Digital converter of the instrument. This means that when the data is despun onboard the spacecraft, the data and the angle used to despin it are slightly out of phase. In order to correct this delay the data are rotated by a phase angle. The delay is equivalent to the rotation outlined below. Bx': cos(delay) * Bx - sin(delay) * By By': sin(delay) * Bx - cos(delay) * By delay : -0.045163 To transform the data to RTN coordinates the following step is taken 5) The data are transformed first into Earth Mean Equatorial(equinox 1950) coordinates. This system is directly supported by the SPICE software provided by the Navigation and Ancillary Information Facility (NAIF) at JPL as inertial coordinate system 'FK4'. The angles required for this transformation come directly from the Galileo Attitude and Articulation Control System (AACS) data. The transformation matrix for this rotation is: -- -- | (cosTsinDcosR - sinTsinR) (-sinDsinTcosR - cosTsinR) cosDcosR | | | | (cosTsinDsinR + sinTcosR) (-sinDsinTsinR + cosTcosR) cosDsinR | | | | -cosDcosT sinTcosD sinD | -- -- where R : Rotor-Right Ascension D : Rotor-Declination T : Rotor-Twist - Rotor-Spin-angle (despun data) Once in an inertial coordinate system, SPICE software provides subroutines which return the transformation matrices to GSE (G_GSETRN), GSM (G_GSMTRN), or RTN (G_RTNTRN) coordinate systems for any ephemeris time. These matrices have been used to perform the coordinate system transformations. The spacecraft/planet (SPK) , leap second (TS), and planetary constants (PCK) kernels required for these transformations have been archived in the PDS by NAIF. These SPICE kernels are available on the CD_ROM which contains the magnetometer data. The SPICE toolkit (software) can be obtained from the NAIF node of the PDS for many different platforms and operating systems. At the time of this archive, the SPICE toolkit was available via an anonymous-ftp site at naif.jpl.nasa.gov

Data Quality: Data quality assessment is a rather vague concept which we will try to address in a somewhat qualitative manner. Each aspect of the data processing sequence can be analyzed to determine its maximum possible error contribution. In theory, these errors could be summed to provide estimates of the maximum error for each data point. We have not taken our error analysis to that level. We believe that our calibrations (sensor geometry and gains) are good enough that they produce a negligible source of data error. In addition, we believe that the coordinate system transformations which are derived from the SPICE kernels and Toolkit are negligible sources of error in the magnetic field vectors. The sources of error which we feel are the most significant are those associated with magnetic sources aboard the spacecraft, especially those with temporal or spacecraft orientation variations. The next greatest contributor of error is the data from the AACS which affects our knowledge of the spacecraft orientation and hence rotates the magnetic field vector. Lastly, telemetry or software errors which produce 'spikes' or bit errors in the data are error sources. Magnetic sources associated with the spacecraft have been identified. Over the long averaging intervals of all the interplanetary cruise data this source of noise is eliminated. There are some time intervals when the zero levels of the spin plane sensors show large variations (1-5 nT) on short time scales (minutes to hours). After a while (hours to days) the offsets return to their nominal levels. The source of these magnetic fields has not yet been identified. The method of removing offsets does not remove these rapidly varying effects. Errors associated with AACS angles have various effects on the data. The rotor right ascension and declination are crucial to the understanding of the spacecraft orientation. Fortunately, these angles are slowly varying and can be interpolated to better than 1 degree of accuracy for long (many hour) time periods except near major spacecraft maneuvers. During some time intervals no AACS data is available. As a result, these intervals can not be transformed from spacecraft coordinates RTN coordinates. During several intervals of the interplanetary cruise the spacecraft lost it's sector data. Sector data gives the magnetometer the neccessary information to despin the magnetic field data spin plane components (Bx_sc and By_sc in spacecraft coordinates). If the magnetic field spin plane components are not despun they will appear as a sin wave. The next step in the processing done onboard the spacecraft is averaging. The data that is not despun will be averaged to zero as you would expect a sin wave to be averaged over many periods. The intervals where sector data was lost have not been removed from the processed IRC dataset. The reason for this is that these intervals can be valuable in helping to determine sensor zero levels. These intervals have been cut from the processed data in RTN coordinates. Below is a list of time where sector data was lost. 1990-Jan-26 19:35:48 to 1990-Jan-31 05:17:29 1991-Jan-16 20:24:58 to 1991-Jan-21 17:36:00 1991-Feb-12 18:47:00 to 1991-Feb-25 18:08:00 1993-Jan-27 09:00:40 to 1993-Jan-30 12:30:27 Ancillary data products: TEMP - The temperature of the outboard sensor is critical to understanding the nature of the spin aligned sensor. Columns in temperature files -------------------------------------------------- name description -------------------------------------------------- time S/C event time (UT) given in the form yyymmddTZ out_temp Temperature at the outboard sensor in_temp Temperature at the inboard sensor